Air conditioning systems for vehicles

ABSTRACT

An air conditioning system for vehicles includes a refrigeration cycle having a variable displacement compressor with a displacement control valve, for controlling a discharge amount of the compressor, so that a pressure difference (Pd-Ps) between a discharged refrigerant pressure (Pd) and a drawn-refrigerant pressure (Ps) equals a set value, an acceleration state detecting device, and an engine rotational speed detecting device. The system further includes a device for determining a condition in which the acceleration state is detected or a condition in which the engine rotational speed equals or is greater than a set value as an elevated compressor load condition, and during the elevated compressor load condition, controlling the discharge amount of the compressor, so that the pressure difference (Pd-Ps) equals or is less than a pressure difference (Pd-Ps) immediately before elevated the compressor load condition. Both superior acceleration performance of a vehicle and driver comfort due to air conditioning may be achieved.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to air conditioning systems for vehicles,and more specifically, to air conditioning systems for vehicles whichimprove the acceleration performance of a vehicle by appropriatelyreducing the displacement of a compressor during acceleration, withoutadversely affecting operation of air conditioning or performance of thevehicle, or both.

2. Description of Related Art

In a known technology, a refrigeration cycle has a variable displacementcompressor, and the power consumed by driving the compressor is reducedby reducing the displacement of the compressor during vehicleacceleration. An amount of power made available to the vehicle byreducing power consumption by the compressor is utilized to improve theacceleration performance of the vehicle. An example of the knowntechnology is described in Japanese Published Patent Application No.JP-A-57-175422.

Nevertheless, there are problems with this known technology for reducinga displacement of a compressor during acceleration. Because thedisplacement of the compressor is controlled at a minimum displacementor a clutch for the compressor is disengaged during acceleration, thecooling ability of the system approaches zero, the temperature of airblown into the vehicle interior increases. Therefore, the driver'scomfort may be adversely affected. On the other hand, if theabove-described compressor control is not performed during acceleration,the power consumption of the compressor increases along with theincrease in engine rotational speed. The acceleration performance of thevehicle may be degraded, and fuel consumption may increase. Because avariable displacement compressor varies a stroke of a piston in order tocontrol a control object, such as a drawn-refrigerant pressure (Ps), ata constant value relative to variations of the engine rotational speed;when the engine rotational speed increases, the piston stroke iscontrolled, so that the torque of the compressor is reduced.Nevertheless, if the drawn-refrigerant pressure (Ps) is a controlobject, when a thermal load of an evaporator is elevated, i.e., when anactual Ps is larger than a target Ps, even if the engine rotationalspeed varies, the compressor is driven at a maximum stroke. Therefore,the power consumption of the compressor increases along with theelevation of the engine rotational speed, and the accelerationperformance of the vehicle may deteriorate significantly.

SUMMARY OF THE INVENTION

Accordingly, a need has arisen to provide air conditioning systems forvehicles, which prevent a compressor from being driven at a maximumdisplacement during acceleration, avoid excessive power consumption bythe compressor during acceleration, and realize improved accelerationperformance of the vehicle and reduced fuel consumption.

Further, a need has arisen to provide air conditioning systems forvehicles, which appropriately reduce the displacement of the compressorduring acceleration to improve the acceleration performance of thevehicle, and, at the same time, which prevent the degradation of thedriver's comfort, by suppressing the excessive elevation of thetemperature of the air blown into a vehicle interior.

In an embodiment of the present invention, an air conditioning systemfor vehicles comprises a refrigeration cycle comprising a variabledisplacement compressor with a displacement control valve forcontrolling a discharge amount of the compressor, so that a pressuredifference (Pd-Ps) between a discharged refrigerant pressure (Pd) and adrawn-refrigerant pressure (Ps) of the compressor equals a set value,means for detecting acceleration of a vehicle, and means for detectingan engine rotational speed of the vehicle. The air conditioning systemfurther comprises means for determining a condition in whichacceleration is detected or a condition in which the engine rotationalspeed is greater than or equal to a set value as an elevated compressorload condition, and during such elevated compressor load condition, forcontrolling the discharge amount of the compressor by the displacementcontrol valve, so that the pressure difference (Pd-Ps) after dischargeis less than or equal to a pressure difference (Pd-Ps) immediatelybefore discharge in the elevated compressor load condition.

This air conditioning system for vehicles further may comprise means forestimating a present value of the pressure difference (Pd-Ps), and forestimating the pressure difference (PD-Ps) during the elevatedcompressor load condition, the compressor may be driven by setting anestimated value of the pressure difference (Pd-Ps) immediately beforesaid elevated compressor load condition as a target control value of thedisplacement control valve.

In this structure, the air conditioning system further may comprisemeans for estimating a torque of the compressor, and the means forestimating the pressure difference (Pd-Ps) may estimate the pressuredifference (Pd-Ps) based on an estimated torque of the compressor or anoutside air temperature or a physical value having a correlation with anamount of air flowing into a heat exchanger provided outside of avehicle interior per a unit time or an engine rotational speed or avehicle running speed, or combinations thereof.

Moreover, in air conditioning systems for vehicles according to thepresent invention, the pressure difference (Pd-Ps) during the elevatedcompressor load condition may be controlled at or less than the pressuredifference (Pd-Ps) immediately before the elevated compressor loadcondition, by controlling the discharge amount of the compressor, sothat power consumed by driving the compressor is equal to or less than aset value. In this case, the set value of the power consumed by drivingthe compressor may be determined with reference to a physical valuehaving a correlation with an accelerator actuating amount and a vehiclerunning speed. Further, a sufficient cooling ability may be determinedfrom a vehicle interior air temperature or an evaporator exit airtemperature or an outside air temperature, or combinations thereof, andthe set value of the power consumed by driving the compressor may bedetermined in accordance with the determined sufficient cooling ability.

In addition, in air conditioning systems for vehicles according to thepresent invention, the above-described means for detecting theacceleration state may detect the acceleration state of the vehicle withreference to at least a physical value having a correlation with anaccelerator actuating amount and a vehicle running speed.

Further, the air conditioning systems for vehicles according to thepresent invention may be configured as a heat pump cycle.

Thus, in air conditioning systems for vehicles according to the presentinvention, the compressor is not always driven at a maximum displacementduring acceleration, excessive power consumption of the compressorduring acceleration may be avoided, and improved accelerationperformance of the vehicle and reduced fuel consumption may be achieved.

Further, because the displacement of the compressor is controlled, sothat the pressure difference between the discharged refrigerant pressureand the drawn-refrigerant pressure is a target value both beforeacceleration and during acceleration, the cooling ability is not reducedduring acceleration, and the comfort level in the vehicle interior maybe maintained.

Further objects, features, and advantages of the present invention willbe understood from the following detailed description of preferredembodiments of the present invention with reference to the accompanyingfigures.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention now are described with reference to theaccompanying figures, which are given by way of example only, and arenot intended to limit the present invention.

FIG. 1 is a schematic diagram of an air conditioning system for vehiclesaccording to an embodiment of the present invention.

FIG. 2 is a graph showing an example of a relationship between adisplacement control signal and a pressure difference (Pd-Ps) in thepresent invention.

FIG. 3 is a graph showing an example of a relationship between a vehiclerunning speed and a power consumed by a compressor when a pressuredifference (Pd-Ps) is constant in the present invention.

FIG. 4 is a time chart comparing respective properties showing anexample of a difference between a case in which the control methodaccording to the present invention is performed and a case in which thecontrol method according to the present invention is not performed.

FIG. 5 is a graph showing an example of determination of an accelerationstate due to an accelerator opening degree according to Example 1.

FIG. 6 is a graph showing an example of a relationship between anaccelerator opening degree and a compressor control method according toExample 1.

FIG. 7 is a flowchart showing an example of a control method accordingto Example 1.

FIG. 8 is a flowchart showing another example of a control methodaccording to Example 1.

FIGS. 9A and 9B are graphs showing examples of a relationship between anaccelerator opening degree and a compressor control method according toExample 2.

FIG. 10 is a flowchart showing an example of a control method accordingto Example 2.

FIG. 11 is a schematic diagram of an air conditioning system forvehicles, showing an example in which the air conditioning system isconfigured as a heat pump cycle.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 depicts an air conditioning system for vehicles according to anembodiment of the present invention. In FIG. 1, in a refrigeration cycle1, a variable displacement compressor 2 is provided which is capable ofchanging its discharge amount. Compressor 2 is driven by an engine for avehicle (not shown), and the drive force of the engine is transmitted tocompressor 2 via an electromagnetic clutch, or the like. Refrigerant iscirculated in the refrigerant tubes in refrigeration cycle 1, and thehigh-temperature and high-pressure refrigerant is compressed bycompressor 2. The refrigerant exchanges heat with outside air in acondenser 3, in which the refrigerant is cooled, condensed, andliquefied. Gas-phase refrigerant and liquid-phase refrigerant areseparated by a receiver drier 4, and the liquid refrigerant is expandedand reduced in pressure by an expansion mechanism 5 (e.g., an expansionvalve). The refrigerant, after being reduced in pressure, flows into anevaporator 6, and the refrigerant exchanges heat in evaporator 6 withair delivered by a blower 11. The refrigerant evaporated in evaporator 6is drawn again into compressor 2 and compressed therein.

Blower 11 is disposed in an air duct 10 through which air for airconditioning of an interior of a vehicle passes, and air drawn from asuction port 9 is delivered to evaporator 6 by blower 11. A portion ofair having passed through evaporator 6 is delivered to a heater unit 8disposed downstream, and the ratio of the amount of air passing throughheater unit 8 to the amount of air bypassing heater unit 8 is adjustedby an air mixing damper 7. Respective air discharge ports 12, 13, and14, such as a DEF-mode, air discharge port, a VENT-mode, air dischargeport, and a FOOT-mode, air discharge port, are provided at downstreampositions in air duct 10, and these ports are opened and closedselectively by respective dampers (not shown).

Compressor 2 is a variable displacement compressor having a displacementcontrol valve (not shown) which controls a discharge amount ofcompressor 2, so that a pressure difference (Pd-Ps) between a dischargedrefrigerant pressure (Pd) and a drawn-refrigerant pressure (Ps) ofcompressor 2 equals a set value. Displacement control signal 16 is sentto the displacement control valve for controlling the discharge amountof compressor 2 and is sent from air conditioning control unit 15. Toair conditioning control unit 15 vehicle signals 17 which are sent froman engine, electronic control unit (ECU) and include a signal from meansfor detecting an acceleration state of a vehicle, a signal of a vehiclerunning speed, a signal of an engine rotational speed sent from anengine rotational speed detecting means, a signal of a cooling fanvoltage, and the like are input to air conditioning control unit 15. Inthis embodiment, a signal indicating an accelerator opening degree is aphysical value corresponding to an accelerator actuating amount. Sensorsignals 18, which include a signal from a sunshine sensor, a signal froma vehicle interior temperature sensor, a signal from an outside airtemperature sensor, a signal from an evaporator exit air temperaturesensor, a signal from a high pressure-side refrigerant pressure sensor,and the like, also are input to air conditioning control unit 15.

In such an air conditioning system thus configured, the control methodaccording to the present invention is accomplished as follows.

As described above, compressor 2 is a variable displacement compressor,which can control the pressure difference (Pd-Ps) between dischargedrefrigerant pressure (Pd) and drawn-refrigerant pressure (Ps) inresponse to the displacement control signal. The relationship betweenthe displacement control signal and the pressure difference (Pd-Ps) maybe as depicted in FIG. 2. As shown by the solid line in FIG. 2, when thedisplacement control signal is increased, the signal transits from aminimum displacement region into a displacement control region, and whenthe displacement control signal is increased further, the signaltransits into a maximum displacement region.

When the above-described displacement control signal varies in theminimum displacement region, regardless of variation of the displacementcontrol signal, the compressor is driven at a minimum displacement. Whenthe displacement control signal varies in the maximum displacementregion, regardless of variation of the displacement control signal, thecompressor is driven at a maximum displacement. When the displacementcontrol signal varies in the displacement control region, the dischargeamount of the compressor is controlled so as to satisfy a target valueof the pressure difference (Pd-Ps) determined in accordance with thedisplacement control signal, and the compressor is driven at anintermediate displacement. In FIG. 2, a displacement control signalindicated by “a” is a value of the displacement control signal at aboundary between the displacement control region and the maximumdisplacement region, and the value of “a” varies depending on thethermal load condition, the compressor rotational speed, and the like.

In the maximum displacement region, as shown in FIG. 2, because thetarget value of the pressure difference (Pd-Ps) is elevated, an actualpressure difference (Pd-Ps) does not reach the target value, and thecompressor is driven at the maximum displacement. When the enginerotational speed elevates during acceleration, because the compressorrotational speed elevates as the compressor is in the maximumdisplacement condition, the power consumption of the compressorincreases significantly.

In the displacement control region, the displacement of the compressoris controlled so as to satisfy the target pressure difference (Pd-Ps).When the engine rotational speed elevates during acceleration, thedisplacement of the compressor is decreased so as not to vary thepressure difference (Pd-Ps), and at the same time, the compressor torquealso decreases. Therefore, although the power consumption for thecompressor increases slightly, the power consumption for the compressordoes not increase significantly due to acceleration.

In the present invention, by driving the compressor by the displacementcontrol signal in the displacement control region during acceleration,an increase in power consumption during acceleration is reduced oravoided, and improved acceleration performance and fuel consumption areachieved.

When the thermal load is elevated, for example, in midsummer, even ifthe compressor is driven at the maximum displacement condition, theevaporator exit air temperature may not decline to a target value. Insuch a case, a displacement control signal, such as that indicated by“b” in FIG. 2, is set in the maximum displacement region, so that thecompressor is driven in the maximum displacement condition, and thecompressor is driven at the maximum discharge amount. When the vehicleis accelerated in such a condition and the engine rotational speedelevates, the power consumption for the compressor increasessignificantly accompanying the elevation of the engine rotational speed,and the pressure difference (Pd-Ps) also increases. Accordingly, in thecontrol method according to the present invention, when the displacementcontrol signal in the maximum displacement region is set beforeacceleration, the compressor is controlled by setting a value smallerthan “a” shown in FIG. 2 as a displacement control signal, so that thepressure difference (Pd-Ps) does not increase during acceleration. As aresult, because the discharge amount of the compressor is reduced, sothat the pressure difference (Pd-Ps) of the compressor does not increasealong with elevation of the engine rotational speed during acceleration,a significant increase of the power consumption for the compressor maybe avoided.

FIG. 3 depicts a relationship between the vehicle running speed and thepower consumption of the compressor at a certain pressure difference(Pd-Ps). When the pressure difference (Pd-Ps) is constant and thevehicle running speed increases, the power consumption of the compressorincreases. Because an amount of air passing through a condenserincreases along with the increase of the vehicle running speed, theability of the air side of the condenser to radiate heat increases. Whenthe ability of the air side to radiate heat increases, a balance pointof discharged refrigerant pressure (Pd) decreases, and because thepressure difference (Pd-Ps) tends to decrease, the compressor torqueincreases in order to maintain a certain pressure difference (Pd-Ps).Therefore, when the pressure difference (Pd-Ps) is maintained atconstant value, the power consumption of the compressor increases.

As shown in FIG. 3, if the pressure difference (Pd-Ps) is constant,because the power consumption of the compressor increases along with anincrease of the vehicle running speed, the power consumption duringacceleration is not less than the power consumption before acceleration.Therefore, if the pressure difference (Pd-Ps) is set to be constantbefore acceleration and during acceleration, the cooling ability doesnot lessen as compared with that before acceleration, and the degree ofcomfort in the vehicle interior may be maintained.

FIG. 4 depicts a time chart showing a case in which the control methodof the present invention is carried out (i.e., the solid line) and acase in which the control method of the present invention is not carriedout (i.e., the dotted line). When the control method of the presentinvention is not carried out, the displacement control signal is set ata value in the maximum displacement region, and the compressor is drivenat the maximum discharge amount. On the other hand, when the controlmethod of the present invention is carried out, by setting thedisplacement control signal, so that the pressure difference (Pd-Ps)does not increase during acceleration, a significant increase of thepower consumption of the compressor is avoided. The power consumption ofthe compressor during acceleration gradually increases because theincrease of the power consumption accompanies the increase of thevehicle running speed. Further, because the evaporator exit airtemperature is controlled to be substantially constant, the degree ofcomfort during acceleration also is maintained.

The displacement control signal at the time of acceleration is set asfollows.

(1) The method for setting the displacement control signal duringacceleration by estimating the pressure difference (Pd-Ps) beforeacceleration:

When the compressor is driven by the displacement control signal in themaximum displacement region as shown by “b” in FIG. 2, because an actualpressure difference (Pd-Ps) is less than a target control value set bythe displacement control signal, the present pressure difference (Pd-Ps)cannot be determined from the displacement control signal. Therefore,when the compressor is driven before acceleration by the displacementcontrol signal in the maximum displacement region, the present pressuredifference (Pd-Ps) is estimated from information other than thedisplacement control signal, and the compressor is controlled, so thatthe pressure difference (Pd-Ps) during acceleration does not exceed theestimated pressure difference (Pd-Ps).

The pressure difference (Pd-Ps) may be estimated by the followingmethod. Although the pressure difference (Pd-Ps) has a high degree ofcorrelation with the torque and the rotational speed of the compressor,because the correlation is influenced by the ability of the air side ofa condenser to radiate heat, the pressure difference (Pd-Ps) may beestimated with a high degree of accuracy by referring to an outside airtemperature and a condenser fan voltage and a vehicle running speed,each of which has a high degree of correlation with the ability of theair side of the condenser to radiate heat. In particular, the pressuredifference (Pd-Ps) may be estimated by the following equation: pressuredifference (Pd-Ps) estimated value=f (compressor torque estimated value,outside air temperature, condenser fan voltage, vehicle running speed,engine rotational speed).

The compressor torque is estimated by the following method. When thecompressor is driven at the maximum displacement, because the compressortorque has a high degree of correlation with a high pressure-siderefrigerant pressure, the torque may be estimated by the followingequation: compressor torque estimated value=f (high-pressure siderefrigerant pressure sensor value, engine rotational speed).

From the calculated pressure difference (Pd-Ps) estimated value, thedisplacement control signal is calculated by a characteristic equationfor the displacement control valve: displacement control signal at thetime of acceleration=f (pressure difference (Pd-Ps) estimated value).

(2) The method for setting the displacement control signal duringacceleration from a restriction value of power consumption for thecompressor:

As shown in FIG. 4, when the compressor is driven at the maximumdisplacement during acceleration, the power consumption for thecompressor increases significantly due to the increase of the enginerotational speed. Consequently, the pressure difference (Pd-Ps) alsoincreases. Therefore, by driving the compressor during acceleration at acondition which is equal to or less than a predetermined value of powerconsumption, the increase of the pressure difference (Pd-Ps) may bereduced or avoided.

During acceleration, a limiting value of the power consumption of thecompressor is determined, and the compressor is driven, so that thepower consumption does not exceed the limiting value. In order tocontrol the displacement of the compressor during acceleration by adisplacement control signal which does not adversely affect the degreeof comfort of the vehicle interior, the above-described limiting valueis determined as a value considering the degree of comfort of thevehicle interior. When the vehicle is accelerated from a stopped state,the power consumption of the compressor may be 1,000 W, when thecompressor is driven at the maximum displacement at an idling conditionof the vehicle. By setting the limiting value of the power consumptionof the compressor during acceleration at, for example, 1,500 W, thecooling ability during acceleration is not less than that during idling.Therefore, the evaporator exit air temperature does not elevate, and thedegree of comfort of a driver is not adversely affected.

With respect to acceleration from a running state, the power consumptionfor the compressor before acceleration is calculated from the torqueestimated value and the engine rotational speed before acceleration. Thecalculated value is set as the limiting value of the power consumptionfor the compressor during acceleration.

The displacement control signal during acceleration is calculated fromthe limiting value of power consumption as follows. First, a compressortorque target value is calculated from the limiting value of powerconsumption and a present compressor rotational speed calculated fromthe engine rotational speed: compressor torque target value=f(60×restriction value of power)/(2π×compressor rotational speed). Fromthe calculated compressor torque target value, a pressure difference(Pd-Ps) target value then is calculated using the aforementionedequation for the pressure difference (Pd-Ps) estimated value: pressuredifference (Pd-Ps) target value=f (compressor torque target value,outside air temperature, condenser fan voltage, vehicle running speed,engine rotational speed).

Acceleration is recognized, for example, by accelerator opening degreeand vehicle running speed. FIG. 5 depicts Example 1 of threshold valuesfor recognizing acceleration. When the accelerator opening degree isgreater than or equal to threshold value 1 and less than threshold value2, acceleration is occurring, and the pressure difference (Pd-Ps) iscontrolled equal to or less than a certain set value. Further, when theaccelerator opening degree is greater than threshold value 2, thecompressor is stopped (OFF) or is driven at the minimum displacement inorder to preferentially accelerate the vehicle. The relationship betweenthe accelerator opening degree and the compressor control due to thethreshold values is shown, for example, in FIG. 6.

Further, a plurality of limiting values of power consumption may begiven as depicted in FIGS. 9A and 9B as in Example 2. More appropriatelimiting values of power consumption may be set relative to acceleratoropening degrees (e.g., accelerator actuating amounts). Moreover, asdepicted in FIGS. 9A and 9B, the setting of the limiting values of powerconsumption may be changed depending upon requirements for coolingability. In particular, when the desired cooling ability is elevated,such as when the temperature of the vehicle interior is high, thesetting of FIG. 9A may be employed. When the desired cooling ability isdepressed, such as when the temperature of the vehicle interior is low,the setting of FIG. 9B for lower limiting values may be employed inorder to improve the acceleration performance more properly. The desiredcooling ability may be determined by referring to an air temperature inthe vehicle interior, a temperature of air having passed through theevaporator, and the like. In particular, when these temperatures aregreater than certain set values, limiting values are determined asdesired when cooling ability is elevated.

Although power consumption limiting values differing from each other areset relative to a plurality of threshold values in FIGS. 9A and 9B, amethod may be employed whereby a power consumption limiting value iscalculated from the accelerator opening degree, and the powerconsumption value is varied continuously in response to variations ofthe accelerator's opening degree.

The flow of the control method during acceleration of theabove-described Example 1 is explained by referring to FIGS. 7 and 8.FIG. 7 depicts a flowchart of the control method of Example 1, wherebythe displacement control signal during acceleration is calculated fromthe pressure difference (Pd-Ps) before acceleration. FIG. 8 depicts aflowchart of the control method of Example 1, wherein the displacementcontrol signal during acceleration is calculated from the limiting valueof power consumption (Pcomp).

(1) When an air conditioning switch is turned to be ON by an operator,the compressor is driven, and the displacement control signal iscontrolled, so that a detected value of an evaporator exit airtemperature sensor becomes a target value (usual control).

(2) A displacement control signal during of acceleration ACC Duty iscalculated at every control cycle by the following equation. In thecontrol shown in FIG. 7, a compressor torque estimated value (Trq′) iscalculated by the following equation:Trq′=f(Pd, RPM)An estimated value (Pd-Ps)′ of pressure difference (Pd-Ps) is calculatedby the following equation:(Pd-Ps)′=f(Trq′, SP, Tamb, CondV, RPM)A displacement control signal during acceleration (ACC Duty) iscalculated by the following equation:ACC Duty=f((Pd-Ps)′)   Equation (1)In the control method shown in FIG. 8, a target compressor torque(TargetTrq) is calculated by the following equation:TargetTrq=f(Pcomp, RPM)A displacement control signal during acceleration (ACC Duty) iscalculated by the following equation:ACC Duty=f(TargetTrq, SP, Tamb, CondV, RPM)   Equation (2)Where,Pcomp=Target power consumption for compressor;Tamb=Outside air temperature;SP=Vehicle running speed;CondV=Condenser fan voltage; andRPM=Compressor rotational speed(3) When the accelerator opening degree is greater than or equal to thethreshold value shown in FIG. 5, acceleration is recognized. Thethreshold values shown in FIG. 5 are threshold value 1 and thresholdvalue 2. When the accelerator opening degree is less than thresholdvalue 1, the compressor is driven, so that the detected value of theevaporator exit air temperature sensor becomes a target value. When theaccelerator opening degree is greater than or equal to threshold value 1and less than threshold value 2, the compressor is driven duringacceleration, such that the displacement control signal (ACC Duty) iscontrolled at the displacement control signal during acceleration.Threshold value 1′ and threshold value 2′ are less than threshold value1 and threshold value 2, respectively, and cessation of acceleration isdetermined, for example, as shown in FIG. 6.

When the displacement control signal (ACC Duty) during acceleration isset, even if the vehicle running speed varies thereafter, and thecalculated value of ACC Duty varies, the compressor is driven at theinitially set value of ACC Duty when the accelerator opening degree isgreater than or equal to threshold value 1′ and less than thresholdvalue 2. In particular, the value of ACC Duty set when accelerationcontrol begins, is maintained until acceleration is complete.

When the accelerator opening degree is greater than or equal tothreshold value 2, the displacement control signal is changed to bezero, and the compressor is driven at a minimum displacement. In thiscase, although the degree of comfort in the vehicle interior is slightlydegraded, because the vehicle requires a significant drive force, thedrive force is given greater importance. Further, when the calculatedACC Duty is greater than the value of the displacement control signalbefore acceleration, the compressor control method for acceleration isnot performed. FIG. 4 shows time chart when the method control of thepresent invention is performed (i.e., the solid line) and when thecontrol method of the present invention is not performed (i.e., thedotted line), as described above.

The displacement control signal during acceleration may be controlled tobe constant from the time at which acceleration begins untilacceleration ends and the power consumption of the compressor may becontrolled to increase gradually, as in the above-described example.Alternatively, ACC Duty may be calculated in response to variation ofthe vehicle running speed also during acceleration and the powerconsumption of the compressor may be controlled to be constant from thetime at which acceleration begins until acceleration ends.

The flow of the control during acceleration in Example 2 is explainedreferring to the flowchart depicted in FIG. 10.

(1) When an air conditioning switch is turned to be ON by an operator,the compressor is driven, and the displacement control signal iscontrolled, so that a detected value of an evaporator exit airtemperature sensor becomes a target value (usual control).

(2) A level of sufficient cooling is determined, and a displacementcontrol signal (ACC Duty) during acceleration is calculated at everycontrol cycle by the following equations:TargetTrq1=f(Pcomp1, RPM);TargetTrq2=f(Pcomp2, RPM);TargetTrq3=f(Pcomp3, RPM);ACC Duty 1=f(TargetTrq1, Tamb, SP, CondV, RPM);ACC Duty 2=f(TargetTrq2, Tamb, SP, CondV, RPM); andACC Duty 3=f(TargetTrq3, Tamb, SP, CondV, RPM).Where,TargetTrq=Target compressor torque;Pcomp=Target power for compressor;Tamb=Outside air temperature′SP=Vehicle running speed;CondV=Condenser fan voltage; andRPM=Compressor rotational speed.

As shown in FIGS. 9A and 9B, there exist a plurality of powerconsumption limiting values, as well as a plurality of threshold valuesof the accelerator opening degree. Therefore, respective values of ACCDuty are calculated at conditions in which respective power consumptionlimiting values are set to be target powers for compressor. Further, asshown in FIGS. 9A and 9B, the respective power consumption limitingvalues are changed depending upon the value of the desired coolingability.

(3) When the accelerator is actuated, the power restriction values(Pcomp 1-3) are set as shown in FIGS. 9A and 9B, and the compressor isdriven by the respective values of ACC Duty corresponding to therespective power consumption limiting values. The values of ACC Duty inthe flowchart depicted in FIG. 10 are calculated, for example, asfollows: ACC Duty 1 is calculated as a displacement control signal whenthe power consumption limiting value of 2,000 W; ACC Duty 2 iscalculated as a displacement control signal when the power consumptionlimiting value of 1,500 W; and ACC Duty 3 is calculated as adisplacement control signal at a time of power consumption limitingvalue of 1,000 W.

Further, when the desired cooling ability is less than a certain setvalue, the values of ACC Duty are calculated, for example, as follows:ACC Duty 1 is calculated as a displacement control signal when the powerconsumption limiting value of 1,500 W; ACC Duty 2 is calculated as adisplacement control signal when the power consumption limiting value of1,000 W; and ACC Duty 3 is calculated as a displacement control signalwhen the compressor is operating at minimum displacement (compressor:OFF). Thus, by reducing the power consumption limiting values, thecompressor may be controlled at a condition in which the accelerationperformance of the vehicle is preferentially improved.

The present invention may operate within a system configured as a heatpump cycle. FIG. 11 depicts an example thereof, and at the same time,FIG. 11 shows the flow of heat medium in a cooling mode, in adehumidification heating mode, and in a heating mode. In FIG. 11, avariable displacement compressor 21, a fist heat exchanger 22 providedinside of a vehicle interior side, a second heat exchanger 23 providedinside of a vehicle interior side, a heat exchanger 24 provided outsideof a vehicle interior, a gas/liquid separator 25, expansion valves 26and 27, and electromagnetic valves 28 and 29 are depicted within the airconditioning system. Other structures are similar to those depicted inFIG. 1. The present invention also is suitable for use in such a system.

Although the control of the compressor during acceleration has beendescribed above, the control method according to the present inventionmay be carried out not only during acceleration, but also for thepurpose of protection of the compressor when the engine rotational speedis greater than a certain set value. For example, when the enginerotational speed exceeds 5,000 rpm, the displacement control signalduring acceleration may be applied. Further, the control methodaccording to the present invention may be applied not only to an airconditioning system for vehicles using freon, but also an airconditioning system for vehicles using a natural-system refrigerant,such as CO₂. Thus, the present invention may be applied to any airconditioning system for vehicles which aims to achieve both goodacceleration performance of a vehicle and a desired degree of comfortdue to air conditioning.

Although embodiments of the present invention have been described indetail herein, the scope of the invention is not limited thereto. Itwill be appreciated by those skilled in the art that variousmodifications may be made without departing from the scope of theinvention. Accordingly, the embodiments disclosed herein are onlyexemplary. It is to be understood that the scope of the invention is notto be limited thereby, but is to be determined by the claims whichfollow.

1. An air conditioning system for vehicles comprising a refrigerationcycle comprising a variable displacement compressor with a displacementcontrol valve for controlling a discharge amount of said compressor, sothat a pressure difference (Pd-Ps) between a discharged refrigerantpressure Pd and a drawn-refrigerant pressure Ps of said compressorequals a set value, means for detecting an acceleration state of avehicle, and means for detecting an engine rotational speed of saidvehicle, said air conditioning system comprising: means for determininga condition in which said acceleration state is detected or a conditionin which said engine rotational speed is greater than or equal to a setvalue as an elevated compressor load condition, and during said elevatedcompressor load condition, controlling said discharge amount of saidcompressor by said displacement control valve, so that said pressuredifference (Pd-Ps) after discharge is less than or equal to a pressuredifference (Pd-Ps) immediately before discharge in said elevatedcompressor load condition.
 2. The air conditioning system of claim 1,wherein said air conditioning system further comprises a means forestimating a present value of said pressure difference (Pd-Ps), and forestimating said pressure difference (Pd-PS) during said elevatedcompressor load condition, said compressor is driven by setting anestimated value of said pressure difference (Pd-Ps) immediately beforesaid elevated compressor load condition as a target control value ofsaid displacement control valve.
 3. The air conditioning system of claim2, wherein said air conditioning system further comprises means forestimating a torque of said compressor, and said means for estimatingthe pressure difference (Pd-Ps) estimates said pressure difference(Pd-Ps) from an estimated torque of said compressor or an outside airtemperature or a physical value having a correlation with an amount ofair flowing into a heat exchanger provided outside of a vehicle interiorper a unit time or an engine rotational speed or a vehicle running speedor combinations thereof.
 4. The air conditioning system of claim 1,wherein said pressure difference (Pd-Ps) during said elevated compressorload condition is controlled at or less than said pressure difference(Pd-Ps) immediately before said elevated compressor load condition, bycontrolling said discharge amount of said compressor, so that a powerfor driving said compressor is equal to or less than a set value.
 5. Theair conditioning system of claim 4, wherein said set value of said powerfor driving said compressor is determined with reference to a physicalvalue having a correlation with an accelerator actuating amount and avehicle running speed.
 6. The air conditioning system of claim 4,wherein a sufficient cooling ability is determined from a vehicleinterior air temperature or an evaporator exit air temperature or anoutside air temperature, or combinations thereof, and said set value ofsaid power for driving said compressor is determined in accordance withsaid determined sufficient cooling ability.
 7. The air conditioningsystem of claim 1, wherein said means for detecting the accelerationstate detects said acceleration state of said vehicle with reference toat least a physical value correlated with an accelerator actuatingamount and a vehicle running speed.
 8. The air conditioning system ofclaim 1, wherein said air conditioning system is configured as a heatpump cycle.
 9. A method for controlling the displacement of a compressorin a vehicle air conditioning system during vehicle acceleration, saidmethod comprising the steps of: detecting a value of an evaporator exitair temperature at air conditioning system start up and assigning saidvalue as a target value; measuring a discharged refrigerant pressure anda drawn-refrigerant pressure; estimating a pressure difference betweensaid discharged refrigerant pressure and said drawn-refrigerantpressure; determining a displacement control signal during vehicleacceleration as a function of said estimated pressure difference; andcontrolling said compressor during acceleration by said displacementcontrol signal, so that a detected value of said evaporator exit airtemperature sensor during acceleration equals said target value.
 10. Themethod of claim 9, wherein said air conditioning system furthercomprises a condenser and a condenser fan, and further comprising thesteps of estimating a compressor torque value; and measuring a vehiclespeed, an outside ambient air temperature, a condenser fan voltage, anda compressor rotational speed and wherein said pressure difference is afunction of said estimated compressor torque, said vehicle speed, saidoutside ambient air temperature, said condenser fan voltage, and saidcompressor rotational speed.
 11. The method of claim 10, wherein saidestimated compressor torque is a function of said discharged refrigerantpressure and said compressor rotational speed.
 12. A method forcontrolling the displacement of a compressor in a vehicle airconditioning system during vehicle acceleration, wherein said airconditioning system further comprises a condenser and a condenser fan,said method comprising the steps of: detecting a value of an evaporatorexit air temperature at air conditioning system start up and assigningsaid value as a target value; measuring a vehicle speed, an outsideambient air temperature, a condenser fan voltage, and a compressorrotational speed; determining a target compressor torque; determining adisplacement control signal during vehicle acceleration as a function ofsaid target compressor torque, said vehicle speed, said outside ambientair temperature, said condenser fan voltage, and said compressorrotational speed; and controlling said compressor during acceleration bysaid displacement control signal, so that a detected value of saidevaporator exit air temperature sensor during acceleration equals saidtarget value.
 13. The method of claim 12, further comprising the stepsof determining a target power consumption for said compressor anddetermining said target compressor torque as a function of said targetpower consumption for said compressor and said compressor rotationalspeed.